yamllint: Make all files in awx pass yamllint

This commit updates all files that weren't passing yamllint for them to
pass.

A new yamllint target has been added. One can run `tox -e yamllint` or
`yamllint -s .` locally to ensure yaml files are still passing.

This check will be enabled in the CI so it can get on every new
contributions, and prevent merging non-compliant code.

Signed-off-by: Yanis Guenane <yguenane@redhat.com>

awx/plugins/inventory/openstack.yml

@@ -1,3 +1,4 @@
+---
 clouds:
   vexxhost:
     profile: vexxhost
@@ -19,6 +20,6 @@ clouds:
       password: stack
       project_name: stack
 ansible:
-  use_hostnames: True
-  expand_hostvars: False
-  fail_on_errors: True
+  use_hostnames: true
+  expand_hostvars: false
+  fail_on_errors: true